On the left is a power transformer. Its primary is assumed to be connected to the AC line. The bottom of the secondary is grounded. The top connects to the anode of a diode. The cathode connects through a resistor to ground. There are two terminals one coming from ground and the other coming from the diode cathode. To the right is a graph having time horizontally and voltage vertically. The graph line starts out at zero time and voltage. It rises tracing out the first half cycle of a sine wave. After returning to zero it remains at zero for the same amount of time it took to rise and fall for the first half cycle. Then it repeats. End verbal description.